Observational studies indicate that mid-ocean ridges are sites where hydrodynamic processes act to enhance turbulent mixing. Previous studies have included measurements from slow-spreading ridges in the Atlantic and Indian Oceans, where the ridges are characterized by very rough topography with deep axial valleys and fracture-zone canyons. The interaction of tides with such topography gives rise to baroclinic currents, which locally enhance the internal-wave energy and increase the potential for instabilities that lead to turbulent mixing. Internal- wave driven mixing of this type has become the paradigm for the occurrence of turbulence in the deep ocean, and has been the basis for parameterizations of diffusivity for use in ocean models. However, recent work has demonstrated that the exceptional levels of turbulent mixing occurring at mid-ocean ridges may also be attributable to low-frequency flow, in particular on slow-spreading ridges where innumerable sills and constrictions support density driven overflows. The main goals of study is to make new observations to document the mixing processes acting on the fast-spreading East Pacific Rise (EPR) which, in comparison to slow-spreading ridges, is associated with much smoother topographic relief. The comparatively less rough topography prevents the considerable enhancement of internal wave energy seen elsewhere, leading previous studies to suggest only weak mixing near the EPR. However, a tracer-release experiment, conducted as part of the NSF-supported LADDER project, demonstrated turbulence levels corresponding to diffusivities of order 10 cm2 s-1, two orders of magnitude above typical thermocline levels. Observations of strong flow, shear, and velocity finestructure suggest that strong turbulence at this site extends beyond a simple bottom boundary layer, and contributes to significant mixing of stratified water above the ridge topography. The sampling program will consist of microstructure and lowered acoustic Doppler profiling, yielding both direct estimates of the turbulent dissipation rates and finestructure parameters. The microstructure measurements will be the first to characterize dissipation levels at any fast-spreading ridge site. We have the unique opportunity to join the final cruise of the LADDER program and contribute new measurements within the framework of existing and funded shiptime. Intellectual Merits: In spite of several decades of research, mixing in the deep ocean remains poorly characterized. In particular, all available measurements on which current roughness- based mixing parameterizations are based, were taken on slow-spreading ridges where different processes have been implicated in the observed high levels of mixing. Therefore, it is entirely unknown to what degree these parameterizations apply on fast-spreading ridges. The proposed microstructure survey will thus significantly extend the available mixing data set from the deep ocean to sites with different topographic characteristics. The measured dissipation rates will be used to improve mixing parameterizations to be used in numerical circulation and climate models. Broader Impacts: In addition to the physical process characterization and resulting implications for process parameterization in models, this study contributes in a fundamental way to the LADDER study of larval dispersion near active hydrothermal sites. The proposed work will also support a graduate student at FSU and a postdoctoral investigator at LDEO.
